Hydrocarbons are an energy source for internal combustion engines, for turbines in jet aircraft, and for other kinds of engines, as well as for other applications that require a source of fuel. For example, hydrocarbon fuels like gasoline are made up of hydrocarbons having about 4-9 carbon atoms in their molecular structure. Kerosene fuels (about 9-15 carbon atoms), jet fuels (9-15 carbon atoms), and diesel fuels (12-20 carbon atoms) are other examples. Lubricant base oils, which have a relatively higher viscosity index than the other examples, typically have 22-35 carbon atoms.
For some time, hydrocarbon fuels, in addition to other petrochemical products, have been obtained from crude petroleum oil through a series of conventional steps. Such steps include, but are not necessarily limited to, distillation followed by additional refining. Attempts are being made, however, to produce hydrocarbon fuels from alternative, renewable sources, including but not limited to feedstocks of biological origin. Moreover, because of their similar chemical properties and functional properties, some hydrocarbon fuels that are from alternative, renewable sources are compatible with and, therefore, acceptable for use with, the kinds of engines for which petroleum-derived hydrocarbon fuels are intended.
More specifically, hydrocarbon fuels, which are from alternative, renewable sources other than petroleum, include those products which are obtained from catalytic isomerisation of linear, olefinic hydrocarbons, as described and taught herein. In some cases, such products are capable of being stored and transported through existing infrastructure (e.g., storage tanks and pipelines) as with petroleum-derived hydrocarbon fuels. This improves the feasibility of using such products as replacements for petroleum-derived hydrocarbon fuels in their applications as fuels.
The words “linear” and “branched” refer to the molecular structure of a hydrocarbon's skeletal chain. In some applications relating to fuels, lubricants, and petrochemical products, the hydrocarbons are linear. In other applications, the hydrocarbons are branched. In some applications, including some in which hydrocarbons are primary constituents for gasoline, kerosene fuels, jet fuels, and diesel fuels, the use of branched hydrocarbons is advantageous over the use of their linear hydrocarbon analogs.
For example, methylhexane is the branched analog of normal heptane. Methylhexane, with an octane number of 44, has a higher octane number than normal heptane, which has an octane number of zero. Branched hydrocarbons with about 9-15 carbon atoms, e.g., when used as kerosene or jet fuel, have lower freezing points and lower pour points than their linear analogs. In certain cold temperature conditions, such properties are advantageous when compared to the properties of linear hydrocarbons having the same number of carbons. Likewise, branched hydrocarbons used in diesel fuels, which are typically 12-20 carbon atoms in length, have lower freezing points and lower pour points than their linear analogs. In the case of lubricant base oils containing 20-35 carbon atoms, branched analogs have lower freezing points and higher viscosity indices than their linear counterparts.
Besides the difference between linear branched, another consideration is whether hydrocarbons contained in fuels, lubricants, and petrochemical products are paraffinic (saturated) or olefinic (unsaturated). Under certain conditions, such as cold temperature, olefinic hydrocarbons provide various advantages over the use of paraffinic hydrocarbons containing the same number of carbon atoms, including but not limited to lower freezing points and lower pour points.
Branched olefinic hydrocarbons, e.g., of the kind which can be used as replacements for petroleum-derived hydrocarbon fuels, do not occur naturally in large supply. However, starting materials for the production of branched olefinic hydrocarbons, for example biomass raw materials, are found naturally in relatively large supply. Accordingly, such starting materials are considered to be an alternative, renewable source of hydrocarbon fuels. For example, the lipid portions of plant oils, animal fats, animal oils and algae oils are a ready source of triglycerides, which are converted to carboxylic acids through methods known to persons of ordinary skill in the art, such as hydrolysis that produces carboxylic acids and glycerine. In turn, various methods are known to persons of ordinary skill in the art for the conversion of carboxylic acids to linear olefinic hydrocarbons, where the carboxylic acid is of the formula R—COOH and R is an olefinic hydrocarbon group. These include hydrodeoxygenation, or (alternatively) decarboxylation, or (alternatively) decarbonylation. If the carboxylic acid starting materials are unsaturated, then the resulting hydrocarbon will be a linear, olefinic hydrocarbon. For example, oleic acid (C17H33COOH) is a monounsaturated fatty acid found in olive oil. The above-mentioned oxygen-removal methods convert oleic acid to heptadecene or octadecene, which are linear, olefinic hydrocarbons.
Another known method is the conversion of linear, paraffinic hydrocarbons to linear, olefinic hydrocarbons by dehydrogenation over a metal catalyst, such as platinum, palladium, or nickel, or a combination of those. Regardless of how they are produced, the isomerisation of linear, olefinic hydrocarbons to branched olefinic hydrocarbons has been performed either at high temperatures or over solid acid catalysts, such as a zeolite or silicoalumina catalyst. However, the isomerisation reactions under such conditions, and/or with the use of such catalysts, generally produce a relatively high percentage of shorter-chain, lower molecular weight hydrocarbon products due to hydrocracking. Accordingly, there is a need for a suitable catalyst and/or catalytic process for the isomerisation of branched olefinic hydrocarbons from linear, olefinic hydrocarbons. There is also a need for such a catalyst and/or catalytic process with suitable selectivity for isomerisation, as opposed to reactions that may be less desirable in certain situations, such as hydrocracking.